1. Field of the Invention
The present invention relates generally to base stations within cellular systems, and specifically to base stations utilizing multi-beam antenna arrays.
2. Background and Objects of the Present Invention
Up until now, cellular time division multiple access (TDMA) systems have conventionally included base stations (BS""s) that utilize either omni or sector antennas (typically 120xc2x0 coverage per sector). These antennas cover the entire cell without any knowledge of the mobile station""s (MS""s) position.
To increase the coverage and capacity of future systems, adaptive antenna systems using multi-beam antenna arrays have been developed. The narrow beams of the antenna array can be used to increase the sensitivity of the uplink, and to reduce the interference in both the uplink and the downlink.
In cellular TDMA systems, for each user, it is only necessary to transmit power in the assigned channel or time slot in the actually active spatial area (cell, sector or beam). Therefore, users in the system are allocated channels to limit (minimize) the mutual interference between themselves and other users. The interference level determines the channel reuse pattern in the cellular grid.
Without changing the cellular BS grid including its sector coverage layout, the interference level in the system can be reduced (less interference is received and spatially spread out) by utilizing beam forming methods with narrow beams based on knowledge of the MS""s positions. This interference reduction can be used either to achieve increased capacity of the system (i.e., decrease the spatial reuse pattern) or to achieve enhanced quality of the actual communication links (i.e., increased data or speech quality to end user).
A number of proposals for cellular systems employing antenna arrays have been published. Reference is made to the following articles and patents: Forssen et al., xe2x80x9cAdaptive Antenna Array for GSM900/DCS1800,xe2x80x9d Proc. 44th Vehicular Technology Conference, Stockholm, June 1994; Hagerman et al., xe2x80x9cAdaptive Antennas in IS-136 Systems,xe2x80x9d Proc. 48th Vehicular Technology Conference, Ottawa,, May 1998; U.S. Pat. No. 5,515,378 to Roy, III et al.; and PCT International Application WO 95/34102 to Forssen et al., each of which is hereby incorporated by reference. For example, Roy, III et al. describes how capacity can be increased by means of allowing multiple MS""s per traffic channel in one coverage area and gives examples of various algorithms. WO 95/34102 to Forssen et al. describes a dual orthogonally polarized microstrip antenna array for use in mobile communication systems.
However, in several TDMA systems, an MS must extract information included within an adjacent time slot or channel, which may be included in a data stream (transmission) aimed for some other MS. For example, in some TDMA systems (IS-136, PDC, GPRS and EDGE), energy must be transmitted on a downlink carrier even though that carrier does not serve an active MS. This may be required to facilitate complexity reduction, to assist the MS in synchronization or to facilitate the scheduling of the air interface recourse. Such requirements limit the ability of cellular TDMA systems to utilize the aforementioned beamforming methods with narrow beams based on the MS""s position in order to reduce the interference level.
Specifically, in IS-136 cellular systems, an MS should be able to use training sequences and color codes included in the surrounding adjacent time slots. These adjacent training sequences may be used to enhance the equalizer function, and therefore enhance the performance of the air interface. No power control is allowed in non-active time slots if one of the slots on the carrier is active, i.e., no power control is admitted on a time slot basis in IS-136 Revision A.
In PDC cellular systems, the MS should be able to measure the received signal strength in adjacent slots with the objective to select the best MS antenna for reception during the active slot (2-branch antenna selection diversity in the MS). A maximum power reduction of xe2x88x928 dB relative to the power level in an active slot is allowed in non-active slots.
Similarly, in GPRS and EDGE cellular systems, an MS must be able to read the uplink state flag transmitted in the downlink to be able to determine if the MS is assigned to use the next uplink time slot for transmission.
As mentioned hereinbefore, a problem arises if one of the above discussed cellular systems utilizes narrow (steerable) beams. Beams that are selected/steered on a time slot basis may spatially steer away for nonactive slots and result in a malfunction.
Therefore, to prevent malfunctions, information can be simultaneously transmitted in several beams of a multi-beam BS antenna configuration. However, due to vector addition of transmitted signals, simultaneous transmission in several beams requires coherent signal paths from the first splitter in the BS up to the antenna array, including feeder cable coherency. The coherency is required for control of antenna pattern characteristics when transmission is directed to more than one beam. Otherwise, the radiation pattern will be uncontrolled and can have significant variations, including possible directions with nulls in the radiation pattern.
Such coherent signal paths are very delicate to achieve in an installed product with several years of expected life time. This kind of solution will require calibration networks that keep track of signals paths and algorithms that compensate for inaccuracies and variations, which both are expensive to introduce in the system.
It is, therefore, an object of the present invention to maintain antenna pattern control when radiating in two or more beams simultaneously.
It is a further object of the present invention to avoid coherency requirements in the feeder cables of a BS antenna arrangement, thereby considerably simplifying and reducing the cost of the system.
The present invention is directed to systems and methods for allowing a base station to simultaneously transmit signals in several beams of a multi-beam antenna configuration. Antenna pattern control is maintained by employing orthogonal polarization orientation for neighboring beams that are used for simultaneous transmission. For example, the two orthogonal polarization directions can be approximately linear polarization slanted xc2x145xc2x0. To be able to transmit simultaneously in an arbitrary combination of more than one beam, the BS antenna array includes a separate feeder cable for each selected combination of beams corresponding to a particular polarization, from one feeder per beam to one feeder with all beams of that polarization. In the overlapping region between two adjacent beams there may be a shift of signal polarization direction, but signal level will remain at an expected level. Accordingly, there does not exist any coherency requirement between different polarizations.